Studies have demonstrated an effect of corticosteroids to change the phospholipid composition of fat cell ghosts prepared from rat epididymal fat pads. An increase in the sphingomyelin content of these membranes has been seen which appears to be secondary to increased synthesis of sphingomyelin following treatment with 8 X 10 to the negative 8th power M dexamethasone. Other more recent studies carried out in this laboratory have demonstrated an effect of insulin to alter the fatty acid composition of phospholipids isolated from similarly prepared fat cell ghosts. Studies are being carried out to determine the mechanisms by which these affects of hormones are brought about to change the composition of the cellular membrane and to investigate the importance of such changes in phospholipid membranes in mediating the physiologic effects of these hormones.